Onion skin (OS) is an environmentally problematic waste that is extensively produced during onion bulb processing. Noteworthy, it contains high amounts of bioaccessible and bioavailable compounds with well documented biological activity.

Polish Researchers published on Food Research International journal a work where they estimated the effect of OS-supplementation and gastrointestinal processing of wheat bread on its potential anticancer activity and ability to modify activity of lipoxygenase (LOX), xanthine oxidase (XO), catalase (CAT) and superoxide dismutase (SOD).

Gastrointestinal processing releases XO and LOX inhibitors and CAT activators from supplemented bread. Most importantly, synergism between these compounds derived from bread and OS was found. Gastrointestinally adsorbed 3% OS-supplemented bread extract exerted a considerably stronger cytostatic and anti-invasive effect on gastric cancer AGS cells than its gastrointestinally digested and buffer masticated counterparts. These effects only partly correlated with the phenolic content of the extracts and their antioxidative activities. Gastrointestinal digestion and adsorption slightly influence the bioactivity of non-supplemented bread extracts. However, the activity of gastrointestinally adsorbed, OS-supplemented wheat bread extract switched from cytostatic and anti-invasive to cytoprotective upon DMSO extraction.

These data directly show a complex network of synergetic and antagonistic effects of single phenolic compounds from onion skin and wheat bread. They indicate that the bioactivity of phenolic compounds and potentially beneficial effects of wheat bread fortification with onion skin may be regulated by subtle interactions between OS and bread phenolic compounds, and food matrix. They are modulated by gastrointestinal processing.

These findings may open perspectives for the introduction of onion skin into wheat bread supplementation which would increase its pro-healthy properties.